Ultrasonic imaging is widely used in many settings, including medical applications. Of particular importance is the use of ultrasound data in the study of tissue. Physicians may use acquired ultrasound data to assist guiding a catheter through a patient's body, or to non-invasively locate vessels prior to IV insertions. The insertion of needles may also be further complicated in some people whose veins are not readily apparent from the skin's surface, for example, in people with thick layers of fat or infants whose veins are small and difficult to detect. In other circumstances involving shock, arteries that need to be accessed to sample blood gasses shrink in response and thus become even more difficult to detect. For some situations, it may be necessary to surgically cut through the body in order to access the desired internal features, a process that is risky and may cause unnecessary delays to treatment.
A typical ultrasonic imaging system includes an array of transducers, a transmit beamformer, and a receive beamformer. The transmit beamformer supplies electrical waveform signals to the transducer arrays, which in turn produce associated ultrasonic signals. Structures in front of the transducer arrays scatter ultrasonic energy back to the transducers, which then generates receive electrical signals. The electrical receive signals are delayed for selected times specific to each transducer so that ultrasonic energy scattered from selected regions adds coherently, while ultrasonic energy from other regions does not. Array processing techniques for processing received signals in this way are known as beamforming and are well known to those in the field.
Current low cost ultrasound imaging devices are either mechanically scanned devices or one dimensional (1D) phased arrays, which are single rows of parallel elements spaced in the azimuthal direction. Each of these can produce a B-scan—an image ‘slice’ that is perpendicular to the face of the transducer and the skin's surface. Beamforming in one dimension can be realized through a relatively straightforward implementation using a linear array of sensors and a beamformiing processor that delays each sensor output by the appropriate amount, weights each sensor output by multiplying by the desired weighting factor, and sums the outputs of the multiplying operation.
While the B-scan can be swept through a volume of tissue and the user can, in principle, visualize the three-dimensional (3D) anatomy, such visualization requires significant training and experience. The C-scan, on the other hand, displays images parallel to the skin's surface, giving the impression of viewing the tissue of interest with the perspective of a clear ‘window’ through the skin. The two systems are illustrated in FIGS. 1(A)-(B) and 2, which respectively show conventional 2D B-Scan and 2D C-Scan operations. Referring to FIGS. 1(A)-(B), having a 1D array 5 in the B-Scan mode, each fired acoustic line 10 in the B-Scan mode results in a long sequence or ‘line’ of image data or points 11. However, referring to FIG. 2, in the C-Scan mode, only one image point 11 in the total sequence of points for each line 10 of firing is essentially obtained.
The C-Scan requires the use of a 2D array 6 processing a 3D volume of data. A beamforming processor becomes much more complex when a 2D sensor array is used. Not only does the number of time delay operations increase as the square of the size of the array, but also the physical structures required to connect each sensor to its corresponding delay becomes increasingly complex. The complexity is increased by the need for continuous operation. Conventional ultrasound systems have beamformers that continuously update beamformed received echo data so that images are displayed in “real time,” or as the echo signals arrive. Since the speed of sound is slow, it becomes necessary for all imaging data along a particular beam line to be continuously formed for that line, and accordingly, it is generally not acceptable to form images with discrete, fixed receive beamforming (or focusing) parameters through multiple signal transmissions.
The vast majority of ultrasound phased arrays that have been researched and used in industry have been 1D transducer arrays. In recent years, there has been some growth in developments involving 1.5D arrays, which consists of a small number of elements (i.e., frequently less than 8) spaced in the elevation direction. Although work has been performed on 2D arrays, progress has generally proven to be extremely challenging. This results from a combination of fabrication difficulties with the transducers, particularly in the electrical connections, and the cost and bulk of the required beamforming hardware. While the hardware challenge diminishes with improvements in integrated circuits, it is evident that achieving a fully populated 2D array is very challenging.
In summary, there is great interest in producing high quality C-Scan images. However, multi-dimensional ultrasonic systems using the 2D arrays required to produce C-scan images are unrealistically high in implementation complexity, size and cost. Moreover, current multi-dimensional ultrasonic systems typically process full 3D volumes of image data. Lacking in particular is a system that efficiently realizes the unique simplifications possible for 2D C-Scans, for which only one plane of data is required for display, as opposed to a full 3D data set normally acquired using 2D transducer arrays. As illustrated, it is evident from the prior art exemplified in FIG. 2 that a significant portion of the time during acquisition in the C-Scan mode is wasted where the received signals make no contribution to the final image. Thus, the conventional multi-dimensional imaging system potentially wastes considerable processing power in producing C-Scan images, and can realize greater efficiency by appreciating that the final 2D image is limited in its cross sectional volume to a single plane of data.